Juli Inkster and Pat Hurst shot 68s to share second place at 205.
Williams fell out of contention with a horrendous 81, while Pat Hurst, who started the day at two under, shot a 75.
Pat Hurst, playing like a cup veteran instead of a rookie, was part of two victories.
Annika Sörenstam and Pat Hurst tied on level par after 72 holes.
This championship was won by Pat Hurst.
Pat Hurst, professional golfer, born in San Leandro on May 23, 1969.
Beaming and wiping away tears at the same time, Pat Hurst walked off the 18th green yesterday with only her second career victory.
But that was not good enough for the lead because Pat Hurst did tie the course record with her eight-under-par round.
Sorenstam would require what she received last week when the second-round leader, Pat Hurst, staggered to the clubhouse with three straight bogeys.
She finished two strokes out of an 18-hole playoff; Sorenstam beat Pat Hurst on Monday.